Just look at the difference it can make...

Actress Naomi Watts (right) has Cool skin with blue undertones which translates into a pink toned complexion. The cool blonde highlights on the right perfectly complement her natural coloring.

When her hair takes on a warm golden color the yellow undertone is
accentuated against Naomi's cool complexion and drains it of natural
color. Of course photographic lights make a difference but even so, her
coloring is 'thrown' and she's added a much stronger lipstick to
try and compensate.

The best hair colors for COOL skin

The best hair color for Cool skins (like Naomi Watts) are those that have Cool undertones because they are naturally designed to work with a Cool skin tone.

Cool skin
has a blue undertone which gives a pink
tinge to your complexion particularly if you are fair.

If your coloring
is stronger then your complexion can sometimes be sallow but this comes into balance when you are wearing Cool colors to complement.

You need Cool hair tones. Yellow tones near to
your face can make you look quite sickly so avoid any golden tones in
your hair which might throw your skin tone out of balance.

Keep cool! Ash blonde highlights, ash
brown low lights, or even a deep burgundy can be introduced to darker
hair. However deep your color you can introduce some stunning sharp
highlights for brightness. If you find it difficult to eradicate brassy
tones from your hair, try a silver shampoo or toner.

The best hair colors for WARM skin

You have a beautiful rich range of Warm hair tones that will flatter and work with your golden toned complexion. There is a strong red influence - from honey to strawberry blonde, light titian red, deep auburn or even red toned chestnut

As a rule of thumb, if you have a fair complexion and light eyes you should select the more gentle lighter tones. With deeper toned skin and/or darker eyes enjoy the deeper tones.

Warm shades can be softly subtle; gold, golden brown, honey and caramel through to the richest copper and auburn.

Nicole Ritchie (above) wears her hair considerably lighter than her deep brunette. Because she has warm skin there is always a yellow slightly 'brassy' look with the blonde that results and her skin tone looks darker by comparison.

She is of course in the business of standing out from the crowd and if you're happy with this effect then it can look very striking. Just be aware of the effect it can cause, dark eyes and eyebrows are actually designed to complement the darker hair.